Millimeter-wave interconnects for superconducting quantum devices

Building extended networks of microwave quantum devices requires efficient coherent links. Current efforts to realize these focus on direct microwave connections by waveguides or cables or on conversion to optical signals and connection by optical fibers. The first approach suffers from losses in the link and requires cooling to millikelvin temperatures. The second needs efficient electrooptical converters which are currently believed to require significant optical pumping powers that impede their operation in cryogenic setups. Here we propose and theoretically analyze an interconnect architecture based on conversion to mm-waves in the 100-1000 GHz range. We study a conversion process based on frequency mixing in nonlinear kinetic inductors and show that its operation requires 9 orders of magnitude less energy per converted qubit than optical conversion. Furthermore, due to the higher energy of mm-wave photons compared to microwaves, links in this frequency domain could be operated at liquid helium temperatures without much added thermal noise. This makes them an appealing alternative to microwave links in cases where cooling of long waveguides to millikelvin temperatures is infeasible.